Deracemisation of amines

ABSTRACT

The present invention relates to a method for the deracemisation or chiral inversion of chiral amines by enzymatic treatment. The method employs a stereoselective enzymatic conversion and either a non-selective or partially selective chemical or enzymatic conversion, simultaneously or sequentially. The invention also provides a method for selecting a suitable enzyme, particularly a suitable amine oxidase, and for the generation of novel enzymes suitable for use in the deracemisation method.

This application is filed pursuant to 35 U.S.C. § 371 as a United StatesNational Phase Application of International Application No.PCT/GB2003/001198 filed Mar. 19, 2003, which claims priority from GB0206415.2 filed Mar. 19, 2002.

The present invention relates to a method for the deracemisation orchiral inversion of chiral amines by enzymatic treatment of a mixture ofenantiomers. The method employs a stereoselective enzymatic conversionand either a non-selective or partially selective chemical or enzymaticconversion, simultaneously or sequentially. The invention also providesa method for selecting a suitable enzyme, particularly a suitable amineoxidase, and for the generation of novel enzymes suitable for use in thederacemisation method.

Enantiomerically pure chiral amines are valuable syntheticintermediates, particularly for the preparation of pharmaceutical targetmolecules. Traditionally, chiral amines have been obtained by separationmethods such as diastereomeric crystallisation using a chiral acid toform a salt of one of the enantiomers, or by kinetic resolution of aracemate using an enzyme to selectively react one enantiomer allowingeasier separation by physical methods such as solvent partitioning orchromatography (1). Whilst such methods can achieve high enantiomericexcess (e.e.), they can yield only a maximum of 50% of the racemicstarting material as the required enantiomer. As with many chiralcompounds, there is an increasing desire to develop synthetic strategiesfor amines that involve either asymmetric approaches or which combineresolution with racemisation of the undesired enantiomer, both of whichcan in principle deliver the product in 100% yield and 100% e.e.Asymmetric methods suggested to date include the use of transaminasesfor conversion of ketones to chiral amines (2, 3, 4). Furthermore thekinetic resolution of amines using lipases such as Burkholderiaplantarii lipase (5, 6) or Candida antarctica lipase (7) has beencombined with racemisation of the unreacted amine either by formation ofan imine (5,6) or by transfer hydrogenation with Pd/C as the catalyst(7).

An alternative approach, which has been termed deracemisation, involvesthe stereoinversion of one enantiomer to the other e.g. using a cyclicoxidation-reduction sequence. To date it has been shown that such asystem can be applied to the preparation of L-α-amino acids by the useof an enantioselective D-amino acid oxidase in combination with anon-selective reducing agent. The original work (8) reported thestereoinversion of D- to L-alanine, albeit in low yield, using sodiumborohydride as the reducing agent. The instability of sodium borohydrideat pH7 precludes its use on a practical scale, and recently we haveshown that deracemisation of amino acids can be made more efficient bythe use of more suitable reducing agents including sodiumcyanoborohydride (13), ammonium formate with Pd/C and also borohydridecomplexes or amine:boranes (14). However, no-one to date hassuccessfully applied a deracemisation method to amines.

SUMMARY OF THE INVENTION

The present invention provides a method for the deracemisation or chiralinversion (generally referred to herein as enantiomeric conversion) ofchiral amines by treatment of a mixture of amine enantiomers with anenzyme capable of catalysing oxidation of the amine in a stereoselectivemanner and, subsequently or simultaneously, treating the mixture with areducing agent. The method is applicable to mixtures of enantiomers invarying proportions, including racemic mixtures, and to conversion(epimerisation) of one single enantiomer to the other. For example themethod is applicable to mixtures of R and S forms of an amine in a ratioof 1:1, 1:2, 1:5, 1:10, 2:1, 5:1, 10:1, 100:1 or other ratios. Theproduct of the enantiomeric conversion is enriched in the desiredenantiomer over the starting material i.e. the desired enantiomer is inenatiomeric excess. Preferably the product comprises a substantiallypure single enantiomer. Thus, in preferred embodiments the enantiomericconversion process of the invention is employed to convert a mixture ofamine enantiomers into a composition consisting essentially of a singleenantiomer, or is employed to convert one substantially pure amineenantiomer into the other, again in enantiomerically pure form.

The reducing agent may be partially stereoselective ornon-stereoselective and may be a chemical reducing agent. Alternativelythe reduction may be enzymatically catalysed. If a chemical reducingagent is to be employed, this may advantageously be selected from sodiumborohydride, sodium cyanoborohydride, amine:borane complexes or atransfer hydrogenation reagent such as ammonium formate with Pd/C. Ifthe stereoselective oxidation and non-stereoselective (or partiallyselective) reduction are performed sequentially, in anoxidation-reduction cycle, the cycle may be performed a plurality oftimes until the desired enantiomeric excess is achieved.

The enzyme capable of catalysing oxidation of the amine in astereoselective manner may be a monoamine oxidase (MAO), particularly amicrobial monoamine oxidase, but any amine oxidase enzyme may beemployed. One MAO which may advantageously be employed in the method ofthe present invention is the Aspergillus niger monoamine oxidase or avariant thereof, for example a variant in which the enzyme differs fromwild-type A. niger MAO by incorporation of one or more mutations,especially in the region of amino acids 25–265 and 334–350, particularlyprefererd are enzymes having a mutation at one or more of amino acids259, 260, 336 and 348, more particularly the mutation N336S or thedouble mutation N336S, M348K.

The present invention also provides a method of directing the evolutionof an originator enzyme by: a) mutating the originator enzyme to createat least one enzyme variant; b) screening said enzyme variant foractivity against a homochiral substrate; and c) selecting one or moreenzyme variants which show greater activity toward the homochiralsubstrate than does the originator enzyme. Optionally steps a), b) andc) may be repeated, using the enzyme variant selected in step c) as anoriginator enzyme. At appropriate stages, the enzyme variant(s) may beassayed against the opposite enantiomer of the substrate, or against amixture of the substrate enantiomers, to confirm enantioselectivity. Inparticular embodiments, the originator enzyme is an oxidase which showsactivity against amines, for example an amine oxidase, especially amonoamine oxidase. The substrate may be any chiral amine which can beoxidised to an imine, including cyclic secondary amines, for exampleamines of Formula I:

In which:

a) R is H or C₁₋₄alkyl; R1 and R2 are independently selected fromsubstituted or unsubstituted C₁₋₁₀alkyl, C₁₋₁₀alkenyl, C₁₋₁₀cycloalkyl,C₁₋₁₀heterocycle, C₁₋₁₀aryl, C₁₋₁₀heteroaryl, C₁₋₄alkyl-aryl,C₁₋₄alkyl-heteroaryl, C₁₋₄alkyl-C₁₋₆cycloalkyl and C₁₋₄alkyl-C₁₋₆heterocycle; or

b) R is H or C₁₋₄alkyl, R1 and R2 together form a substituted orunsubstituted C₁₋₁₀cycloalkyl ring system or C₁₋₁₀aryl ring containingone or more heteroatoms; or

c) R and R1 together form a substituted or unsubstituted C₁₋₁₀cycloalkylor C₁₋₁₀aryl ring system which may contain one or more heteroatoms andR2 is defined as in a) above.

As used herein, the terms “halo” or “halogen” refer to fluorine,chlorine, bromine and iodine.

As used herein, the term “alkyl” refers to a straight or branchedhydrocarbon chain containing the specified number of carbon atoms. Forexample, C₁-C₃alkyl means a straight or branched hydrocarbon chaincontaining at least 1 and at most 3 carbon atoms. Examples of alkyl asused herein include, but are not limited to; methyl, ethyl, n-propyl,i-propyl.

As used herein, the term “cycloalkyl” refers to a fully saturatedhydrocarbon ring containing the specified number of carbon atoms. Theterm “cycloalkyl” encompasses single and bicyclic ring structures.Examples of cycloalkyl as used herein include, but are not limited tocyclohexyl, cyclopropyl.

As used herein, the term “aryl” refers to an unsaturated which may besaturated or unsaturated hydrocarbon ring containing the specifiednumber of carbon atoms. The term “aryl” encompasses single and bicyclicring structures. Examples of cycloalkyl as used herein include, but arenot limited to phenyl, naphthyl.

Where cycloalkyl or aryl ring systems contain one or more heteroatoms,these are selected from N, S or O, preferably N. Thus, the terms“heterocyclic” and “heteroaryl” refer to cycloalkyl and aryl groups,respectively, which contain up to three heteroatoms selected from N, Sor O, preferably N.

Where one or more of R, R1 and R2, or a ring formed therebetween, aresubstituted, one to three substituents may be present and are selectedfrom halogen, hydroxy, C₁₋₃alkyl, C₁₋₃alkenyl, C₁₋₃alkoxy, nitro,nitrile and CONH₂.

Conveniently, the target substrate can be used for evolution of enzymeswith improved activity and enantioselectivity for that particularsubstrate.

The invention therefore provides a method of improving the catalyticactivity, and optionally also the enantioselectivity, of an amineoxidase, especially a monoamine oxidase, by directed evolutioncomprising mutation of the enzyme and selection of a mutant havingimproved activity against a homochiral substrate. The use of an enzymevariant, selected by such a method, in a method for the deracemisationor epimerisation of amines is also provided.

The wild-type amino acid sequence of A. niger MAO is set out in SEQ. IDNo. 1. In one embodiment the invention provides a variant of the A.niger MAO in which the enzyme differs from wild-type A. niger MAO in theregion of amino acids 334–350, particularly amino acid 336 and/or aminoacid 348, more particularly by incorporation of the mutation N336S orthe double mutation N336S, M348K. The invention thus provides a variantof the A. niger MAO having the amino acid sequence set out in SEQ. IDNo. 2 and, in a further embodiment, a variant of the A. niger MAO havingthe amino acid sequence set out in SEQ. ID No. 3.

Throughout the specification and the claims which follow, unless thecontext requires otherwise, the word ‘comprise’, and variations such as‘comprises’ and ‘comprising’, will be understood to imply the inclusionof a stated integer or step or group of integers but not to theexclusion of any other integer or step or group of integers or steps.

A chiral compound will have two or more enantiomers which arestereochemically dissimilar. A composition which contains more than oneenantiomer of a chiral compound is referred to as a “racemic mixture” ifit contains the enantiomers in equal or substantially equal amounts. Bycontrast a composition is “homochiral”, “enantiomerically pure” or a“substantially pure single enantiomer” if it contains a singleenantiomer, substantially free of the corresponding enantiomer, orconsists essentially of one enantiomer in the absence of the other. By‘substantially free’ is meant no more than about 5% w/w of thecorresponding enantiomer, particularly no more than about 3% w/w, andmore particularly less than about 1% w/w is present. “Enantioselective”and “stereoselective” are used herein interchangeably and refer to thetendency of a reaction to favour one enantiomer of a chiral compoundover the other. “Partially stereoselective” (or “partiallyenantioselective”), “non-enantioselective” etc. shall be understoodaccordingly.

The invention will now be described in more detail, with reference tothe accompanying drawings and sequence listings, in which:

FIG. 1 shows the reaction scheme for the oxidation-reduction cycle whichresults in deracemisation;

FIG. 2 shows diagrammatically the assay used to detect enzymes havingthe desired enantioselective amine oxidase activity;

FIG. 3 shows the results of 27 enzymes selected from the detection assayof FIG. 2, assayed against L-AMBA and D-AMBA;

FIG. 4 shows the substrate specificity of the N336S, M348K mutant A.niger monoamine oxidase;

FIG. 5 shows the enantioselectivity of the N336S, M348K mutant A. nigermonoamine oxidase;

SEQ ID NO:1 shows the amino acid sequence of the wild-type Aspergillusniger monoamine oxidase enzyme (NB we found the sequence to differ by 4amino acids from that reported by Schilling & Lerch (10,11). Thesechanges most likely represent errors in the original DNA sequencing; twoof these differences are also noted by Sablin (12).);

SEQ ID NO:2 shows the amino acid sequence of a variant monoamine oxidaseenzyme generated by directed evolution in the following Examples. Themutation N336S is shown in bold face type and underlined;

SEQ ID NO: 3 shows the amino acid sequence of a variant monoamineoxidase enzyme generated by site directed mutagenesis of the N336Smutant enzyme of SEQ ID NO: 2. The mutations N336S and M348K are shownin bold face type and underlined.

Herein we report a significant new extension of deracemisation, byapplying the method for the first time to the deracemisation of chiralamines. The reaction follows the general scheme shown in FIG. 1. Oneenantiomer is converted to the other because the enzyme reactspreferentially with one of the two chiral forms, leaving the otherunreacted. The enzyme reaction produces the achiral imine, which doesnot have a chiral centre at the nitrogen-bearing carbon. Non-selectivereduction of the imine results in the creation of a 1:1 mixture of amineenantiomers (if a partially selective reducing agent were employed, anunequal mixture of amine enantiomers would be formed). As this processis allowed to undergo a number of cycles, the amine approachesenantiomeric purity (assuming a sufficiently enantioselective amineoxidase). The yield can approach 100% If the imine can be efficientlyreduced before undergoing hydrolysis to the ketone.

Amine oxidases have been classified into two groups, namely Type I(Cu/TOPA dependent) and Type II (flavin dependent) (15). In thecatalytic cycle of the Type I enzyme, the intermediate imine remainscovalently bound to the protein, which mitigates against intervention atthis stage in the reaction to reduce the imine back to the amine. TheType II enzymes have been extensively studied from mammalian sources(9), however microbial sources of Type II enzymes are poorly documentedand indeed at the outset of our work there were no reports ofenantioselective transformations. Schilling et al., (10, 11) reportedthe cloning and expression of a Type II monoamine oxidase fromAspergillus niger (MAO-N) and subsequently Sablin et al., (12) purifiedthe enzyme to homogeneity and carried out substrate specificity andkinetic studies. The enzyme was reported to have high activity towardssimple aliphatic amines (e.g. amylamine, butylamine) but was alsoactive, albeit at a lower rate, towards benzylamine.

Our studies have revealed that the A. niger MAO-N enzyme possesses verylow, but measurable activity towards L-α-methylbenzylamine with evenslower oxidation of the D-enantiomer. Thus the enzyme is partiallyenantioselective. We have carried out in vitro evolution (16, 17) togenerate new amine oxidase enzymes having improved catalytic activityand enantioselectivity over the wild-type A. niger MAO-N enzyme. We havealso demonstrated the applicability of the evolved mutants inderacemisation reactions.

To generate a large library of enzyme variants we used a mutator strainfor random mutation of a plasmid containing an insert encoding thewild-type A. niger amine oxidase enzyme. The E. coli XL1-Red mutatorstrain (Stratagene) has been employed previously for in vitro evolutionexperiments and has the advantage that all parts of the plasmid aresubject to mutation (cf. error prone PCR where only the gene of interestis mutated) which can result in improved production of enzyme as well aschanges to enzyme activity and/or specificity. By carrying out multiplecycles with the mutator strain followed by transformation of the plasmidlibrary into E. coli BL21 cells, we were able to generate a library ofaround 10⁶ variants.

Having generated the mutant enzymes, we assayed them for amine oxidaseactivity. Amine oxidases are typical of all members of the oxidasefamily in that they evolve hydrogen peroxide as a by-product. Many ofthe reported assays for oxidase activity exploit the production ofhydrogen peroxide, especially coupled with peroxidase in the presence ofa substrate that upon oxidation yields a highly coloured product. Wefirst used aminoantipyrine/tribromohydroxybenzoic acid as the substratefor the peroxidase, which yielded distinct pinkish-red colonies whichcould be easily visualised. However the coloured product is relativelysoluble and hence colour from the active colonies rapidly diffusedgiving high background intensity. This problem was overcome by switchingto 3,3′-diaminobenzadine (DAB) as the substrate which gave rise to adark pink, insoluble product resulting in both very high definition andcontrast of the active colonies. It should be noted that thishigh-throughput screen should be generally applicable to other oxidaseenzymes in addition to amine oxidases.

The library was plated out directly onto nitro-cellulose filters onagar-plates (around 3,000 colonies per plate) and a sub-set of theplates taken through the screening protocol (see FIG. 2). Each filterwas transferred to a petri-dish and stored at −20° C. for 24–72 h inorder to partially lyse the cells. Thereafter, each plate was treatedwith a cocktail containing both the assay mixture and also 2% agarose at60° C. The plates were incubated at 37° C. for 24–48 h after which theywas inspected for positive clones which were then removed and plated athigher dilution to isolate pure colonies.

The result of screening a subset (ca. 150,000 clones) of the initiallibrary led to the identification of 35 clones with improved activitytowards L-AMBA compared to the wild-type enzyme. Each of these cloneswas grown on a small scale and assayed against both L- and D-AMBA ascell-free extracts resulting in the best 27 clones which were selectedfor further study. Each of these 27 were assayed against L-AMBA andD-AMBA, the results of which are presented in FIG. 3. Finally, themutant enzyme clone which showed greatest improvement over the wild-typeenzyme in terms of its selectivity, and activity, towards L- versusD-AMBA was selected for more detailed examination. Further mutation wasthen introduced to boost expression.

It is known that the codons used in the DNA and RNA to encode aminoacids show a certain redundancy. Thus, a given amino acid may be codedfor by more than one 3-base codon. However, a given organism may morecommonly use one of the alternative codons than the other(s) for a givenamino acid. This has implications if a heterogeneous nucleic acid isintroduced into a host organism for expression. Sometimes the codonsused by the originator organism are the “less preferred” codons for thehost, which can result in difficulties in expression such as a reducedlevel of expression. Similarly, certain amino acids may be less commonlyrepresented in the proteome of one organism when compared to another. Itmay sometimes possible to boost the levels of heterologous expression ina host cell by using an alternative nucleic acid sequence in whichcodons and/or amino acids which are less preferred by the host areexchanged for more preferred alternatives. The present inventionprovides an isolated nucleic acid which encodes a monoamine oxidaseenzyme which is a variant of the A. niger MAO in which the codons forR259 and R260 are optimised for expression in a heterogeneous host cell.More particularly, when the expression of the MAO is intended to takeplace in E. Coli, the arginine amino acids at these positions arereplaced by alternative amino acids of a similar charge and size. Thesemutations may in addition to mutations which alter the catalyticactivity and/or enantioselectivity of the mutant enzyme. In a furtherembodiment, we provide an enantioselective monoamine oxidase enzymewhich is a variant of the A. niger MAO and which differs from wild-typeA. niger monoamine oxidase by incorporation of the mutations N336S andone or more of M348K, R259L and R260L.

The mutant N336S identified below may be taken as the basis for“hotspot” mutation in which further mutations are introduced in theamino acids surrounding position 336. Mutations may be made bysite-directed mutagenesis, by the construction of chimeric recombinantenzymes using a “cut and paste” methodology employing restrictionenzymes, or by other means well known to workers skilled in the art.This method is used to identify further mutants with enhanced activityover the N336S mutant, such as the N336S, M348K double mutant identifiedherein.

EXAMPLE 1

Preparation of Mutated Plasmid DNA Library by E. coli XL1-Red MutatorStrain.

The MAO gene from Aspergillus niger cloned in pET3a was obtained from B.Schilling (11). The gene was amplified by pfu Turbo DNA polymerase(Stratagene) using primers designed according to E. coli codon usage.The PCR product was subcloned into the pET16b vector (Novagen). Thisconstruct (MAOpET16b) was submitted for mutagenesis by the E. coliXL1-Red mutator strain.

E. coli XL1-Red mutator strain competent cells were obtained fromStratagene. Competent cells were transformed as described in theStratagene protocol. 700 μl of transformed cell suspension(Transformation 1) was inoculated in 20 ml of LB medium (tryptone, 10g/L; yeast extract, 5 g/L; NaCl, 10 g/L; pH7) with ampicillin (100μg/ml; LB Amp) and grown for 18 h in a 50 ml Falcon tube in an incubatorshaker at 37° C. These growing conditions were used throughout theexperiment.

20 μl of growing culture was inoculated in 10 ml of fresh LB Amp andgrown for 24 h. The plasmid was purified (Qiagen Plasmid DNA MiniprepKit) from 1 ml culture (pMAO2) and used for the second transformation(Transformation II) of the mutator strain. The total transformed cellsuspension was inoculated in 10 ml of LB Amp and grown for 24 h. Theplasmid was purified from 1 ml of culture (pMAOretr1.1). 100 μl ofTransformation II growing culture was used to inoculate 10 ml LB Amp.The culture was grown for 24 h, the plasmid purified (pMAOretr1.2) andused for Transformation III. Total transformed cell suspension (1 ml)was inoculated in 10 ml LB Amp, grown for 24 h and the plasmid purified(pMAOretr2.1). 100 μl of Transformation III growing culture was used toinoculate the next 10 ml of LB Amp, grown for 24 h and the plasmidpurified from 1 ml culture (pMAOretr2.2) and used for Transformation IV.Total suspension of transformed cells (1 ml) was used to inoculate 10 mlof LB Amp, grown for 24 h and the plasmid purified from 1 ml culture(pMAOretr3.1). 100 μl of Transformation IV growing culture was used toinoculate fresh 10 ml LB Amp, grown for 24 h and the plasmid purified(pMAOretr3.2).

Collected pools of mutated plasmid DNA (pMAOretr2.2, pMAOretr3.1,pMAOretr3.2) were used to transform E. coli BL21(DE3) to express mutatedMAO genes and detect activity towards L-α-methylbenzylamine (L-AMBA)oxidation.

EXAMPLE 2

Screening for MAO Mutants

The plate assay method (FIG. 2) was used to identify MAO mutants withactivity towards L-AMBA. More specifically, E. coli BL21(DE3)transformants (2500 colonies per plate) were plated on HiBond-C Extra(Amersham Pharmacia) membrane placed on an LB Amp agar plates andincubated for 24 h at 37° C. Membranes containing the colonies werepulled from the plates, kept at −20° C. for 24 h and incubated withassay mixture at room temperature for 24 h.

Assay Mixture:

-   -   1 tablet of DAB (3,3′-diaminobenzidine, Sigma, D4418)    -   1 ml of K phosphate buffer (1M, pH7.6)    -   30 μl L-AMBA (10 mM)    -   30 μl horseradish peroxidase (Sigma) 1 mg/ml    -   10 ml 2% agarose (Bio-Rad)    -   Water up to 20 ml

Positive clones were subjected to a second round of screening (100–200colonies per plate) to confirm activity.

EXAMPLE 3

Activity Studies

a) Clone Selection

27 positive clones identified in the plate assay as having improvedactivity towards L-AMBA compared to the wild-type enzyme were grown on asmall scale and assayed against both L- and D-AMBA. 10 ml LB Amp wasinoculated with a single colony of E. coli BL21 (DE3) transformed withthe protein expression vector pET16b harbouring the gene of interest andcultured in a 50 ml Falcon tube with agitation for 24 h. At the end ofincubation 1 ml of cell culture was centrifuged, the pellet wasresuspended in 1 ml of 25 mM potassium phosphate buffer pH7.6 and 0.1 mlwas used to perform a hydrogen peroxide formation assay using both L-and D-AMBA as substrates.

Assay mixture: (Manfred Braun et al, Applied Microbiology andBiotechnology (1992) 37:594–598)

-   -   5 ml phosphate buffer (1M, pH7.6)    -   500 μl 2,4,6-tribromohydrobenzoic acid (2% in DMSO),    -   37.5 μl 4-aminoantipurine (1M)    -   32.5 μL or D-AMBA (final concn 5 mM)    -   Water up to 50 ml        To 895 ul Assay Mixture was Added:    -   5 μl HRP(Sigma, P6782) (1 mg/ml)

Sample (100 μl) was added and absorbance at 510 nm measured after 24hours against a control without sample. The absorbance results are shownin FIG. 3.

Several of these 27 clones appear to be expression mutants, as theydemonstrate increased protein expression when visualised onpolyacrylamide gels (data not shown). The plasmid of the best mutantidentified, which appeared not to have increased expression but whichshowed greatest improvement over the wild-type enzyme in terms of itsselectivity, and activity, towards L- versus D-AMBA, was grown on alarger scale and the enzyme purified. The wild-type enzyme was alsopurified by the same protocol and the two enzymes compared for substratespecificity and enantioselectivity.

The mutated gene was also sequenced and the sequence is shown in SEQ IDNO: 2. There was a single amino acid change from the wild-type enzymewith serine replacing asparagine at position 336.

b) Growth and Comparison of Mutant and Wild-Type Enzymes

Growth and Purification of MAO Mutant Expressed in E. coli BL21 (DE3)

LB medium (6×300 ml) containing ampicillin (100 μg.ml⁻¹) in 1 L baffledflasks was inoculated with a single colony of monoamine oxidase mutantfrom an LB agar plate. Cultures were incubated at 30° C. for 22 hours(OD₆₀₀˜3.6) then harvested and washed with phosphate buffer (50 mM, pH8) to yield a yellow-brown pellet (11.2 g).

The pellet was resuspended in Tris/HCl buffer (25 mM, pH 7.8, 30 ml) andsonicated on ice (30 s on, 30 s off for 10 minutes). The suspension wasthen centrifuged (20,000 rpm, 4° C.) until clear supernatant wasobtained and the supernatant dialysed against Tris/HCl buffer (25 mM, pH7.8). The cell free extract was filtered through 0.45 μm sterilemembrane and chromatographed on a QSepharose anion exchange column.Fractions were assayed using colorimetric hydrogen peroxide based assayand active fractions were stored at −80° C. The active fraction has aprotein content of 1 mg.ml¹, and a specific activity of 0.193 U.mg⁻¹against amylamine.

Chromatography Conditions:

-   -   Column=HiFlow QSepharose 26/10    -   Buffer A=Tris/HCl (25 mM, pH 7.8)    -   Buffer B=Tris/HCl (25 mM, pH 7.8)+1 M NaCl    -   Flow rate=4 ml.min⁻¹    -   Fraction collect=10 ml    -   Column wash=2 CV 100% buffer A    -   Elution=10 CV 100% buffer A to 100% buffer B    -   Column clean=4 CV 100% buffer B        Assay Mixture:    -   5 ml phosphate buffer (1 M, pH 7.6)    -   500 μl 2,4,6-tribromo-3-hydroxybenzoic acid (2% in DMSO)    -   37.5 μl 4-aminoantipyrine (1.5 M)    -   30 μl amine substrate (final concentration 0.015–5 mM)    -   44.4 ml water        Assay Conditions:    -   990 μl assay mixture    -   5 μl horse radish peroxidase (1 mg.ml⁻¹)    -   10 μl enzyme

The spectrophotometer was blanked against assay mixture and HRP. Enzymewas added and the absorbance at 500 nm measured at 3s intervals for a 10minute period.

Results:

Wild-type Enzyme Mutant Enzyme Substrate K_(m) mM k_(cat) min⁻¹ K_(m) mMk_(cat) min⁻¹ L-AMBA ND 0.17 0.4 8.0 D-AMBA ND 0.01 ND 0.08 benzylamineND 371 ND 196 amylamine — 1000 0.4 116

The K_(m) values were calculated using ‘KaleidaGraph for Windows’(Synergy Software). For calculation of the k_(cat) the active enzymeconcentration was determined by estimating the FAD content from theabsorbance at 458 nm using an extinction coefficient of 11 mM⁻¹ cm⁻¹(see ref 12).

The data reveal that the activity of the mutant amine oxidase towardsL-AMBA (kcat=8.0 min−1) is 47 fold higher than the wild-type (kcat=0.17min−1). Moreover the selectivity of the mutant for the L-enantiomerversus D-AMBA (ca. 100:1) has also been increased relative to the wildtype enzyme (ca. 17:1). Thus the outcome of the in vitro evolutionexperiments has been to simultaneously improve both theenantioselectivity and catalytic activity of the enzyme. For comparison,the activity towards the best substrate for the wild-type enzyme, namelyamylamine, and also benzylamine, is presented. The substantialimprovement in activity and selectivity of the mutant was confirmed bychiral HPLC (Chiralcel CrownPak CR+) in which after 24 h completeoxidation of the L-enantiomer was apparent whereas there was nodetectable conversion of the D-enantiomer.

EXAMPLE 4

Deracemisation Reaction

Using DL-AMBA as the substrate, in the presence of the mutant MAO-N, arange of reducing agents were screened (sodium borohydride, catalytictransfer hydrogenation, amine:boranes). This screen identifiedammonia:borane as the optimal reagent which gave a 77% yield of D-AMBAwith an enantiomeric excess =93%. Greater optical purities of theproduct (up to 99% e.e.) could be achieved although at the expense ofyield.

More specifically, MAO mutant N336S (100 μl of 0.193 U.ml⁻¹=0.02 U) wasadded to a solution of DL-AMBA (0.13 μl, final concentration=0.8 mM) andammonia-borane complex (10 μl of 4M solution, final concentration=80 mM,100 eq) in phosphate buffer (400 μl, 20 mM, pH8). A 10 μl aliquot wasdiluted in 990 μl perchloric acid, pH 1.5 and analysed by HPLC. Thereaction mixture was incubated at 30° C. and the reaction monitored byHPLC at regular intervals until no further reaction was observed.

Yield: D-AMBA=77%, e.e.=93%

Analytical Conditions:

-   -   Column=Chiralcel CrownPak CR+    -   Mobile phase=100% perchloric acid, pH 1.5    -   Flow rate=0.8 ml.min⁻¹    -   Detection=200 nm    -   Temperature=25° C.    -   r.t. (L-AMBA)=12.8 min    -   r.t. (D-AMBA)=16.5 min

EXAMPLE 5

Stereoinversion Reaction

We also carried out the stereoinversion of L- to D-AMBA (18% yield, 99%e.e.) and showed that under identical conditions there was no conversionof D- to L-AMBA.

More Specifically, MAO mutant N336S (100 μl of 0.193 U.ml⁻¹=0.02 U) wasadded to a solution of L-AMBA (0.13 μl, final concentration=0.4 mM) andammonia-borane complex (10 μl of 4M solution, final concentration=80 mM,200 eq) in phosphate buffer (400 μl, 20 mM, pH8). A 10 μl aliquot wasdiluted in 990 μl perchloric acid, pH 1.5 and analysed by HPLC. Thereaction mixture was incubated at 30° C. and the reaction monitored byHPLC at regular intervals until no further reaction was observed.

Yield D-AMBA=18%, e.e. >99%:

NB: no reaction was observed after 24 hours when D-AMBA was used assubstrate under identical reaction conditions.

Analytical conditions:

-   -   Column=Chiralcel CrownPak CR+    -   Mobile phase=100% perchloric acid, pH 1.5    -   Flow rate=0.8 ml.min⁻¹    -   Detection=200 nm    -   Temperature=25° C.    -   r.t. (L-AMBA)=12.8 min    -   r.t. (D-AMBA)=16.5 min

Analysis of the HPLC chromatograms suggests that the yield in thederacemisation reactions is prevented from reaching 100% due to theformation a by-product with longer retention time as the reactionprogresses. We are currently optimising the deracemisation protocol toachieve the exquisite levels of yield and selectivity previouslydemonstrated for the deracemisation of α-amino acids.

Abbreviations:

-   MAO—monoamine oxidase.-   AMBA—α-methylbenzylamine-   TBHBA—2,4,6-tribromo-3-hydroxybenzoic acid-   AAP—4-aminoantipyrine-   HRP—horse radish peroxidase type VI from bovine liver-   LB—Lubria Bertani-   r.t.—retention time

EXAMPLE 6

Substrate Specificity of the N336S Mutant Enzyme

The activity of the mutant enzyme described above was studied inrelation to a variety of amine substrates. Assay conditions wereessentially as set out in Example 3. The substrates tested were:

Results:

Relative Sample Rate (S)-α Methylbenzylamine 1 (R)-α Methylbenzylamine *(rac)-α Methylbenzylamine * (S)-4-Methylphenylethylamine 0.28(R)-4-Methylphenylethylamine * (S)-α-Methyl-4-nitrobenzylamine 0.91(R)-α-Methyl-4-nitrobenzylamine 0.06 (S)-4-Bromo-α-phenylethylamine 0.24(R)-4-Bromo-α-phenylethylamine * (rac)-4-Bromo-α-phenylethylamine *(S)-1-4-Methoxyphenylethylamine 0.81 (R)-1-4-Methoxyphenylethylamine0.13 (S)-MTQ 0.67 (R)-MTQ * (rac)-MTQ 0.13 (S)-3-Methyl-2-butylamine 5.4(R)-3-Methyl-2-butylamine * (S)-3-3-Dimethyl-2-butylamine 1.4(R)-3-3-Dimethyl-2-butylamine * 2-Methylcyclohexylamine 0.43N-heptylamine 12.1 N-amylamine 25.2 Hexylamine 18.6Bis(α-methyl)benzylamine * 1,3-Dimethylbutylamine 1.81,2,3,4-Tetrahydro-1-naphthylamine 0.13 * No measurable rate

EXAMPLE 7

Creation of a Further Mutant Clone

The first published sequence of wild-type A. niger monoamine oxidaseshowed a lysine at position 348. Some of the mutant enzymes generatedabove and initially tested in Example 3(a) were thought to be doublemutants because they had a methionine at this position. However, whenthe wild-type sequence was checked (resequenced), the wild-type aminoacid at position 348 was shown to be methionine (as set out in SEQ IDNO:1). In order to elucidate the impact (if any) of a mutation at thissite, site-directed mutagenesis was performed on the wild-type enzyme tointroduce the M348K mutation. It was noted that the identity of theamino acid at this position influenced the efficiency of expression,with lysine giving rise to increased expression without altering thecatalytic activity of the enzyme (absolute activity in U/ml is greaterthan wild type, but correction for the amount of protein gives a valueof k_(cat) which is the same as for the wild-type enzyme). Site directedmutagenesis was then performed upon the N336S mutant described above togenerate a second mutation (M348K), thus combining the catalyticenhancement of the N336S mutation with an increase in expression. Theamino acid sequence of the double mutant is set out in SEQ ID NO: 3.

EXAMPLE 8

Substrate Specificity of the N336S, M348K Mutant Enzyme

The activity of the double mutant enzyme described above was studied inrelation to a variety of amine substrates. Assay conditions wereessentially as set out in Example 3. The substrates tested and therelative rates of conversion (compared to L-AMBA set at 100) are shownin FIG. 4. For some substrates, both enantiomers were tested assubstrates. The enzyme appears to demonstrate enantioselectivity inevery case, with activity against the opposite enantiomer beingundetectable in several cases (the enantioselectivity data is shown inFIG. 5).

EXAMPLE 9

Application of Expression Mutant R260K

The mutation R260K was found to be in low occurrence codon for arginine.The sequencing results revealed that all of the “expression” mutants hadthe same mutation at position 260 wherein arginine was replaced bylysine.

The wild type MAO-N amino acid sequence has two arginines at position259 and 260 both encoded by the codon (AGG), which has a low occurrencein E. coli genes (4%). Lysine and arginine are both basic amino acidsand hence replacement of one by the other should not affect the chargeof the protein, however the replacement of a low frequency codon for ArgAGG (4%) by a high frequency codon for Lys AAG (22%) results in improvedprotein expression in E. coli. Thus we decided to replace the codons ofboth Arg 259 and Arg 260 with the codon CGT (38%) by site directedmutagenesis to evaluate the effect upon the expression level of MAO-N.An alternative approach would be to create a “silent” mutation byaltering the AGG codon to a different codon which still encodes arginine(AGA, CGT, CGC, CGA, CGG).

MAO WT and MAOArg259/260 were partially purified from recombinant E.coli BL21(DE3) harbouring a pET16b carrying the corresponding mao-ngene. Cell free extracts were prepared by sonication and the specificactivities of MAO WT and MAOArg259/260 towards AA were measured for boththe soluble and insoluble fractions. (Table 3)

TABLE 3 Specific activities towards AA in partially purified MAO WT andMAOArg259/260 towards amylamine. Protein Soluble Insoluble concentrationfraction fraction Total Enzyme mg · ml⁻¹ U · mg⁻¹ U · mg⁻¹ activity UMAO WT 1.18 0.23 0.33 0.57 MAOArg259/260 1.30 0.46 0.56 1.2

To confirm the increased level of the R260K mutant expression, Cell freeextracts of MAO WT, mutant 4 (R260K) and MAOArg259/260 were obtained andassayed towards L-AMBA. This was achieved by incubating 50 μl of eachsample for 240 minutes and specific activities were determined. (Table4)

TABLE 4 Soluble ΔAbs fraction Enzyme 510 nm U · mg⁻¹ MAO WT 0.06 3.6 ×10⁻⁵ MAOArg259/260 0.12 7.0 × 10⁻⁵ MAO mutant 0.16 5.7 × 10⁻⁵ (R260K)

In conclusion, we have significantly extended the deracemisationstrategy by applying the method for the first time to the deracemisationof chiral amines. In so doing we have successfully achieved the‘directed evolution’ of an enzyme in order to meet the specificrequirements of a novel biotransformation. Another interesting aspect ofthe present work is the identification of a highly enantioselectivemutant by using a single enantiomer substrate in the screen (L-AMBA).There has been much discussion concerning the need for trulyenantioselective screens in which racemates are used, thereby mimickingthe real-life situation found in a kinetic resolution in which the twoenantiomers compete for the enzyme. It may be that if one is able toscreen truly large and diverse libraries of variant genes (e.g. 10⁶)then it is possible to select for enantioselectivity in the mannerdescribed herein, using inherently simpler screens.

The application of which this description and claims form part may beused as a basis for priority in respect of any subsequent application.The claims of such subsequent application may be directed to any featureor combination of features described herein. They may take the form ofproduct, composition, process or use claims and may include, by way ofexample and without limitation, one or more of the following claims:

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1. An enantioselective monoamine oxidase (MAO) enzyme having the aminoacid sequence of SEQ ID NO:2.
 2. An enantioselective monoamine oxidaseenzyme which is a variant of wild-type the A. niger MAO, where saidvariant differs from wild-type A. niger monoamine oxidase byincorporation of the mutation: N336S.
 3. An enantioselective monoamineoxidase enzyme which is a variant of wild-type A. niger MAO, where saidvariant differs from wild-type A. niger monoamine oxidase byincorporation of the mutations: N336S and M348K.
 4. An enantioselectivemonoamine oxidase enzyme which is a variant of wild-type A. niger MAO.where said variant differs from wild-type A. niger monoamine oxidase byincorporation of the mutations: N336S and one or more of M348K, R259Land R260L.
 5. An isolated nucleic acid molecule which encodes anenantioselective monoamine oxidase enzyme which is a variant ofwild-type A. niger MAO, where said variant differs from wild-type A.niger monoamine oxidase by incornoration of the mutation N336S and inwhich the codons for R259 and R260 are optimised for expression in aheterologous host cell.